Maximum quality, 100%-size JPEGs from PS end up as lower-res images in Keynote.

OK, so I'm a hardcore Photoshop user CS6 — designing user interfaces.
I use Keynote to make semi-interactive user experience demos — web/app walkthroughs etc.

Which often means making images larger than the slide dimensions, so I can animate mock scrolling actions etc.

Here's the problem:

If I export a top-quality JPEG from Photoshop at 1:1 size match to the Keynote, eg. 1024 x 768, I get a beautiful image that remains crisp and true all the way to the iPad presentation (via iCloud)

If, however, I create a "deeper" JPEG, say 1024 x 2200, so I can animate the image in Keynote to mimic the scrolling down of a web page, Keynote seems to always "crush" the quality of the image, as if I'd saved it at medium or even low JPEG settings (which obviously, I did not).

I've been messing with this and outputting different image dimensions — all at 1024px wide (the width of my slide), but gradually increasing the pixel height of the export:

1024 x 768 = 100% beautiful

1024 x 1500 = 100% beautiful

1024 x 1800 = 100% beautiful

1024 x 2048 = 100% beautiful (Attached image below)

1024 x 2049 = Compressed and unuseable (Attached image below)

So MY deduction is that Keynote will only tolerate an image with a pixel height UP TO twice its width. ONE PIXEL more than that, and it turns on the ol' hidden crusher and compresses the file.

Segment of 1024 x 2048:

Segment of 1024 x 2049:

If anyone has any insight into this phenomenon, I'd be very grateful as it's very restricting.

Many thanks in advance